1. Field of the Invention
The present invention relates to a driving control device and an image forming apparatus that includes the driving control device.
2. Description of the Related Art
An image forming apparatus is known, in which toner images carried by a plurality of image carriers are transferred onto a surface of an endless belt that moves endlessly or onto a recording sheet that is held on the endless belt in a superimposed manner. Such an image forming apparatus is disclosed in, for example, Japanese Patent No. 3658262. The image forming apparatus includes a transfer unit in which a belt member in the form of an endless intermediate transfer belt moves endlessly while being supported by a driving roller and a driven roller. The transfer unit transfers toner images of different colors that are formed on a plurality of photosensitive elements as image carriers onto the intermediate transfer belt in a superimposed manner, thereby obtaining a full color image. An image forming apparatus that uses a direct transfer method is also disclosed in Japanese Patent No. 3658262. In the direct transfer method, instead of an intermediate transfer method that uses the intermediate transfer belt, the toner images on the photosensitive elements are superimposed and transferred onto a recording sheet held on a surface of an endless sheet-transporting belt. The above methods to transfer toner images formed on the respective image carriers onto the surface of the belt member or onto the recording sheet on the belt member in a superimposed manner is called the tandem method.
In the image forming apparatus that uses the tandem method, toner images are superimposed, but are often displaced with each other due to a speed variation of the belt member. At the time of superimposing transfer, if a speed variation occurs in the belt member, the toner images on the image carriers are transferred at mutually displaced positions. Uneven thickness of the belt member in a circumferential direction is likely to result in the speed variation of the belt member. If a portion of the belt member having comparatively greater belt thickness is wound on the driving roller that drives the belt member, a belt moving speed increases. If a portion of the belt member having comparatively smaller belt thickness is wound on the driving roller, the belt moving speed decreases. Due to this, a speed variation occurs during a single rotation of the belt member. In the belt member that is manufactured using a centrifugal casting method, decentering of a metal mold for casting the belt results in occurrence of uneven thickness such that a phase lag of 180 degrees occurs between maximum thickness portions and minimum thickness portions in a single belt rotation. In the uneven thickness mentioned earlier, a characteristic of the speed variation per single belt rotation is a sine curve of one cycle.
The image forming apparatus disclosed in Japanese Patent No. 3658262 uses the belt member in which predetermined positions in the circumferential direction are marked and a mark sensor detects the marks at predetermined endless movement positions of the belt member. Furthermore, based on the marks, a belt thickness variation pattern of one rotation in the circumferential direction is prior stored, and based on a mark detection timing and the belt thickness variation pattern, a driving speed of the belt member is adjusted to curb the speed variation of the belt member due to uneven thickness.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-228777 includes a thickness detector that detects a thickness of the belt member based on a distance displacement of the belt member from the sensors and an electrical resistance. The thickness detector detects the thickness of the belt member while causing the belt member to move endlessly. After storing the belt thickness variation pattern in a belt circumferential direction based on the result of the detection in a data storing unit, the driving speed of the belt member is adjusted, thus curbing the speed variation due to uneven thickness of the belt member.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2005-115398 includes an encoder that detects a rotation angular displacement or a angular velocity of support rollers by which the belt member is supported. Based on a result of the detection, the speed variation pattern per one rotation of the belt member is measured. Next, based on the speed variation pattern that is stored in a data storage unit, the driving speed of the belt member is adjusted. Thus, speed variation due to uneven thickness of the belt member is curbed.
The thickness of the belt member changes due to expansion and contraction of the belt member resulting from environmental changes such as temperature and humidity, and belt elongation, belt wear, and tear due to passage of time. The thickness variation pattern per one rotation of the belt member changes due to a change in the thickness of the belt member. However, in the image forming apparatus disclosed in Japanese Patent No. 3658262, the change in the thickness variation pattern cannot be reflected in the adjustment of the driving speed of the belt member. However, in the image forming apparatuses disclosed in Japanese Patent Application Laid-open No. 2001-228777 and 2005-115398, even if the thickness of the belt member changes due to environmental change, the change in the thickness can be reflected in the adjustment of the driving speed of the belt member in the following manner. That is, the thickness variation pattern and the speed variation pattern (hereinafter, collectively called “variation pattern”) are measured and updated at each predetermined timing. Thus, the speed variation due to uneven thickness of the belt member can be reliably curbed.
However, in the image forming apparatuses disclosed in Japanese Patent Application Laid-open No. 2001-228777 and 2005-115398, depending on the timing to measure the variation pattern, an image formation command from a user cannot be received until measurement of the variation pattern is completed. Due to this, a waiting time period of the user increases, thus causing inconvenience to the user.